Coatings of W--Co--Cr--C are used in applications where both superior wear and corrosion resistance are required. A typical composition for these coatings comprises about 8 to 10 weight percent cobalt, about 3 to 4 weight percent chromium, about 4.5 to 5.5 weight percent carbon and the balance tungsten. These coatings can be successfully applied to various substrates, e.g., iron base alloy substrates, using known thermal spray techniques. Such techniques include, for example, detonation gun (D-Gun) deposition as disclosed in U.S. Pat. Nos. 2,714,563 and 2.950,867, plasma arc spray as disclosed in U.S. Pat. Nos. 2,858,411 and 3,016,447, and other so-called "high velocity" plasma or "hypersonic" combustion spray processes.
Although coatings of W--Co--Cr--C have been employed successfully in many industrial applications over the past decade or more, there is an ever increasing demand for even better coatings having superior strength and wear resistance.
It is also desirable to deposit these coatings at faster deposition rates than heretofore possible and thereby coat the substrates more economically. A problem with fast deposition rates, however, has been that high residual stresses tend to build up inside the coating. Unless the coating has sufficiently high strength to resist these stresses, the coating may crack and even spall.
In the copending application Ser. No. 546,480 of J. E. Jackson et al., filed on Oct. 28, 1983, and assigned to the common assignee hereof, there is disclosed a new family of W--Co--Cr--C coatings containing a high content of cobalt and having an improved strength. These coatings are ideally suited for use on gate valves, for example, in the petrochemical industry for handling highly corrosive fluids under high hydraulic pressures. Although these coatings are tough and strong enough to resist the high residual stresses that develop as a result of fast deposition rates, the wear resistance of the coatings is no better than that of conventional coatings.
As is generally known, coatings of W--Co--Cr--C derive their wear resistance from the presence of complex carbides of W, Co, and Cr. Corrosion resistance is derived, on the other hand, from the presence of chromium. The chromium content is actually a compromise between that which is ordinarily required for corrosion resistance and that which interferes with or diminishes the wear and mechanical properties of the coating.